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X-RAYS FOR DIAGNOSIS

X-rays are electromagnetic radiation like light and radio-waves. But visible light is of wavelength about 500 nm, while X-rays have a wavelength of about 0.1 to 0.01 nm - ie. about 1000 times smaller. This means that their frequency is 1000 times greater, and, since E = hf, it also means that their photon energy is 1000 times greater, which gives them penetrating ability useful in medical diagnosis.
All e.m. radiation is produced either by accelerating or decelerating charges, or by charged particles such as electrons falling to lower energy states in atoms. X-rays can be produced by electrons in both ways:
(a) Continuous spectrum Free electrons can be evaporated off a piece of electrically heated metal - thermionic emission. They are then accelerated in a vacuum through a potential difference of about 70 kV between this metal cathode and a tungsten target anode. When they hit the anode they collide with electrons in the tungsten and decelerate. Their kinetic energy is converted to the energy of an X-ray photon; hence hf_max = QV. (See Fig 1.) This is called braking radiation . Since the vacuum is not perfect the electrons will hit the anode with various energies up to QV, and they will suffer various reductions in energy - depending on the nature of their collision in the tungsten. Hence the spectrum of X-ray energies emitted by a stream of incident electrons will be continuous, like - say - that emitted by an incandescent metal. The maximum intensity of radiation is emitted in a region rather below the maximum photon energy. (The detailed shape of the graph does not require explanation at this level) Fig 2. (Tungsten is used as a target material because it has an unusually high melting temperature 3377 ^oC (99% of the incident energy appears as heat in the target), and a proton number of 77 - which is better large because the massive, larger, and more charged, atom is a better target for the incident electrons and therefore produces a greater intensity of X-rays.)